1. Field of the Invention
The invention relates to an electro-optical material based on liquid crystal dispersed in a polymer. This use of this type of material is particularly promising in the field of optical shutters and notably in the field of display screens.
2. Description of the Prior Art
The following is the electro-optical effect used in these composite materials: a film of composite material is generally sandwiched between two transparent, conductive electrodes. At rest, this film is a light-scattering film because of the differences in indices of refraction within the material. When a voltage is applied to the terminals of this film, the oblong-shaped liquid crystal molecules get oriented along their main axis parallel to the electrical field. By adjusting the ordinary index of the liquid crystal to the index of the polymer, a medium with a homogeneous index is obtained, and the film becomes transparent. It is thus possible to pass from a light-scattering state to a transparent state.
The use of these liquid crystal/polymer composite materials offers numerous advantages over other techniques of display. Indeed, these composite materials:
benefit from the ease of implementation of the polymers, which enables their easy deposition as thin films with controlled thickness on large surfaces; PA1 do not necessitate any surface treatment of the substrates that form the screen; PA1 work without any polarizer (whence an increase in gain); PA1 show an electro-optical effect with an angle of view of 150.degree.. PA1 the first technique consists in making an emulsion of a liquid crystal (insoluble in water) in an aqueous phase that is polymer-rich (hydrosoluble polymer or latex) and then evaporating the water to solidify the emulsion; PA1 the second technique consists in making a homogeneous mixture of a liquid crystal and of a polymer or precursor (that may contain a solvent of the liquid crystal which is also a solvent of the polymer), and then in bringing about the separation of the liquid crystal either by polymerizing it (in the case of a precursor) or by cooling it or again by evaporating the solvent if any. PA1 the geometrical factors of the composite material (the thickness of the film and the size and shape of the cavities filled by the liquid crystal); PA1 the dielectric properties of the polymer (resistivity and dielectric constant); PA1 the optical properties of the polymer (index of refraction); PA1 the nature of the anchoring of the liquid crystal to the polymer. PA1 the making of a porous film of polymer on a substrate (S), said polymer having reactive functional groups (Y); PA1 the chemical conversion of the reactive functional groups (Y) of the polymer chains located at the interface between the polymer (P) and the pores of the network, so as to obtain polymer chains having functional groups (Y) in volume and polymer chains having functional groups (X) on the surface; PA1 the inclusion of liquid crystal molecules (.chi.L) in the pores of the film. PA1 the film is a porous film of polymer, the liquid crystal molecule (.chi.L) being in the pores of the film; PA1 the polymer (P) comprises functional groups (Y) and, at the interface between the polymer and the liquid crystal, functional groups (X).
The existing techniques used for the application of these composite materials are notably of two types:
In these composite materials, it has been shown that parameters such as the control voltage (needed for the flip-over of the liquid crystal molecules), the response time, the contrast and the phenomena of hysteresis depend directly on:
Now, all these variables depend partially or exclusively on the nature of the polymer. Thus, for a given polymer, it is most usually necessary to bring about a compromise among the different parameters (such as control voltage, contrast etc.) to be obtained.